Deep-discharge battery separator

ABSTRACT

A tubular, rigid, porous, ceramic separator for a rechargeable, deep-discharge battery assembly, the separator having a porosity greater than 40%. A plurality of battery cells, each embodying such separators, are assembled with a common terminal to form the positive electrode in a motive traction battery.

This application claims the benefit of U.S. Provisional Application No.60/005,834, filed Oct. 23, 1995, entitled DEEP-DISCHARGE BATTERYSEPARATOR, by Thomas N. Gardner, Alvin J. Salkind, John L. Stempin andDale R. Wexell.

RELATED APPLICATIONS

This application is related to Ser. No. 08/491,766 filed Jun. 19, 1995and Ser. No. 08/506,713 filed Jul. 26, 1995, both filed in the names ofJ. L. Stempin, R. L. Stewart, and D. R. Wexell and assigned to theassignee of this application now issued as U.S. Pat. Nos. 5,514,494 and5,554,464, respectively. The former is directed to a rigid, porous,ceramic battery separator having a porosity of 40-90%, a pore size of0.1-25 microns, a thickness of 1-12 mm and resistance to acid attack.the latter is directed to a rigid, ceramic separator for a rechargeablebattery assembly, the separator having a honeycomb structure in whichopen cells are separated from adjacent cells by thin porous wallsrunning lengthwise of the honeycomb and the open cells and pores beingfilled with electrolyte to permit in ion flow between batteryelectrodes.

FIELD OF THE INVENTION

Deep-discharge batteries and tubular, porous, ceramic battery separatorsfor such batteries.

BACKGROUND OF THE INVENTION

The oldest and best known type of rechargeable battery is the lead-acidbattery. The present invention is primarily concerned with heavy dutybatteries of this type designed to provide deep-discharge. Inparticular, it is directed to tubular separators for use in suchbatteries.

Heavy duty, lead-acid batteries are commonly used as the power source infork trucks, golf carts, other electrically powered road and servicevehicles and in marine applications, such as boats, ships andsubmarines. Both tubular and flat plate battery designs are used forthis type battery. The present application is concerned with the formerdesign, that is, the tubular design. In particular, it is directed attubular separators for use as a component in such deep-dischargebatteries.

Presently, the positive plates in a tubular battery consist of a seriesof parallel, porous tubes. Each tube has a centralized lead conductorsurrounded by active material. The tubes are presently made from woven,braided, or felted fibers. Such materials are resistant to acid attackand to the oxidizing environment of lead-acid batteries. However, theylack structural integrity and do not lend themselves to convenient,automated manufacture.

An integrated cell for a heavy duty, deep-discharge battery normallyconsists of several tubes. These may be employed individually, or,alternatively, they may be joined together in what is known as agauntlet construction. This construction integrates several individualtubes into a single structure. The tubes are mounted at their base witha plastic bottom bar. Conventional negative electrodes and separatorsmay be used to complete the tubular design battery.

The important consideration for deep-discharge, deep-cycling batteriesfor traction applications is maximum cycle life with high energydensity. However, light weight is not always desirable in certainapplications. For example, a forklift battery must be heavy, because theweight of the battery is generally used to counterbalance the payload.The life of these batteries is increased by employing thick plates withhigh paste density, a high temperature cure with high humidity, lowelectrolyte density, high quality, organic-based separators, and one ormore layers of glass fiber matting.

The flat pasted (Faure) positive plate is typical for deep cyclingbatteries in the United States. However, some cycling batteries in theUnited States, and most cycling batteries in the rest of the world, arebuilt with tubular or gauntlet type positives. the tubular constructionminimizes both grid corrosion and shedding of active material.Flat-pasted negative plates are used in conjunction with thesepositives, and the cells are of the outside-negative design. Batteriesfor traction and deep-cycle applications have similar performance witheither pasted or tubular positive plates. However, the tubular orgauntlet plates show lower polarization losses because of the largeractive surface area, better retention of the positive active material,and reduced loss on idle or stand.

The present invention provides an extruded, ceramic, tubular separatorto replace the current woven fiber gauntlet and the glass mat separator.

SUMMARY OF THE INVENTION

The present invention resides in a porous, rigid, ceramic, tubularseparator for a tubular, deep-discharge (deep cycling) battery. Itfurther resides in a deep-discharge tubular battery embodying suchtubular bodies as separators.

PRIOR ART

Prior art known to applicants and deemed to be relevant is providedseparately.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a side view in cross-section of a single, tubular battery cellin accordance with the invention.

FIG. 2 is a partial side view of an integrated positive cell for adeep-discharge battery in accordance with the invention.

FIG. 3 is an exploded side view depicting a modified form of theinvention.

FIG. 4 is a perspective view of the modified form of FIG. 3.

DESCRIPTION OF THE INVENTION

Our present invention adopts the basic structural features of currentheavy duty, deep-discharge, tubular cell batteries. In such batteries,the positive plates consist of a series of parallel porous tubes. Eachtube has a centralized lead conductor surrounded by active material. Thetubes are presently made from woven, braided, or felted fibers which areresistant to the acid electrolyte and to the oxidizing environment oflead-acid batteries. The tubes may be used individually. Alternatively,they may be stitched together, (gauntlet construction) to produce asingle structure with several tubes. The tubes are sealed at their basewith a plastic bottom bar. Conventional negative electrodes andseparators are used to complete the tubular design battery.

The present invention is primarily concerned with providing an improvedtubular separator for a deep-discharge battery. Therefore, reference ismade to the prior art for details regarding construction and operatingcharacteristics for this type of battery. A typical description may befound at pages 219-227 of a text by Clive D. S. Tuck entitled "ModernBattery Technology" and published by Ellis Horwood (1991).

Our invention arises from the concept of employing porous, ceramic,tubular bodies as separators in deep-discharge batteries. These ceramicseparators are substituted for the fibrous tubes and porous separatorscurrently used for this purpose in deep-discharge batteries. Theyprovide for ease of assembly compared with the non-rigid, tubularseparators.

In producing such ceramic, tubular separators, we adopt and modifypractices and procedures from the art of ceramic body extrusion. Thus, abatch of raw materials is mixed to provide a homogeneous mass ofsuitable viscosity for extrusion. This mass is fed through an extruderwith a die designed to extrude a continuous length of ceramic tubing.

Our preferred ceramic materials for ceramic separator purposes arecomposed of alumina or mullite alone or mixed with each other. Sourcesof these materials in powder form are mixed with methylcellulose, adispersant, graphite and water to form extrudable mixtures. The mixturesare extruded in tubular form having a desired wall thickness, and arefired to produce porous, tubular separators.

Chemical durability is necessary since the separator is exposed to theelectrolyte. The industry test used for a lead-acid battery separatorinvolves exposure of the material to sulfuric acid solution of 1.28specific gravity for 72 hours at 70° C. The material must exhibit aweight loss that is less than 5% to be acceptable. For convenience incoordinating testing, we have adopted a more stringent test thatinvolves exposure to 40% sulfuric acid for 96 hours at 95° C. Further,we have required that weight loss in this more stringent test not exceedabout 2%.

Finally, a porous material must have good wickability. This is a measureof the ability for the pores to take up electrolyte by capillary action.For example, a glass fiber mat separator typically will allow a sulfuricacid electrolyte to rise to a height of 7.5 cm (3") in a period of 3minutes.

The ceramic material in the walls of an extruded separator has aninherent porosity of about 30-40%. However, a greater value is generallyconsidered necessary to provide a sufficiently low impedance to producea viable battery. A porosity greater than about 50% is preferred.

In order to enhance the porosity in an extruded ceramic, the batchprepared for extrusion may incorporate a combustible or evanescentfiller in amounts up to about 75%. We prefer powdered graphite as thefiller. When a body is extruded, it is fired to remove the filler,thereby enhancing the porosity of the body to values greater than 40%,preferably greater than 60%.

An electrical circuit, such as a battery, contains resistance (R),capacitance (C) and inductance (L). An impedance Z is defined tocalculate the overall retarding effect on current of components with R,L or C. The impedance is critical to operation of a battery andexpresses the system's slow response to a stimulus, namely the effect oncurrent flow upon application of a stimulus (charging and discharging).Power (E in watts) in the direct current (DC) mode of batteries isdefined as the product of the current (I in amperes) and the impedance(Z in ohms) for the DC components of the battery only. Impedanceinvariably reduces the theoretical voltage of a battery to a lowerworking voltage.

Successful battery performance requires ability to accept and maintain acharge. To this end, the impedance value must be relatively low. Duringbattery formation a total energy input is targeted in terms of a fixedampere-hours/pound (Ah/lb). This input must occur with the voltage inany cell not exceeding a certain level. Normal practice is to provide atotal energy input of 185 Ah/lb while maintaining the impressed voltagebelow 2.7 volts. If the porosity of a separator is too low, theimpressed voltage will exceed the permissible limit. This necessitatescutting back the energy input level, a situation that interferes withproper formation of the battery and increases manufacturing time andcost.

Heavy duty, lead-acid batteries are used as the power source in forktrucks, golf carts, other electrically powered road and service vehiclesand marine applications. The primary requirement for these heavy dutybatteries is to have good cycling capability. Most types of tractionbattery are guaranteed for 1200 cycles or five years service. Two typesof battery design are widely used for this application, i.e. tubular andflat plate construction.

The positive plates in the tubular cell consist of a series of parallelporous tubes each having a centralized lead conductor surrounded byactive material. The tubes are presently made from woven, braided, orfelted fibers which are resistant to acid and the oxidizing environmentof lead-acid batteries. The tubes may be used individually or stitchedtogether (gauntlet construction) to produce a single structure withseveral tubes. The tubes are sealed at the base with a plastic bottombar. Conventional negative electrodes and separators are used tocomplete the tubular design battery.

In the tubular battery cell design, the extruded, ceramic, tubularseparators replace the current woven fiber gauntlet and the glass matseparator. An electrode construction is formed in the center of theextruded ceramic body. The exterior of the extruded body acts as theseparator between the electrodes. The ceramic gauntlet/separator may beprocessed in a one piece construction to provide an active positiveelectrode using existing tubular battery technology.

The ceramic tubular construction can also be made in two pieces withchannels which are pasted with active materials. The pasted halves arejoined together and a centralized lead conductor spline incorporated toproduce a tubular positive electrode/separator. Plastic holders may beused to cap and fasten the tubular body together. The ceramic body istailored to the desired porosity and impedance to produce a battery withthe desired deep-discharge characteristics.

Ceramic separators provide significant advantages for use in heavy dutycommercial batteries. The materials can be processed into a variety ofshapes and sizes with a wide range of porosity and pore sizes. These canbe tailored to each battery's requirements.

The materials are strong and do not shred or break apart during normaluse of the battery. The materials do not break while under compressionand prevent active material from falling off the electrodes, therebyextending the life of the battery. The materials exhibit tortuousporosity which deters the ability of dendrites from moving through theseparator and shorting the cell. The strength of the ceramic separatorsmakes the materials ideal for automated processing and for use in eithervertical or horizontal positions. The ceramic tubes also supplystructural strength to the battery.

The use of the ceramic, tubular construction potentially revolutionizesthe fabrication process of deep-discharge batteries. It significantlymechanizes the process of manufacturing, and improves performance byincreasing energy and power densities.

FIG. 1 in the accompanying drawing is a side view in cross-section of asingle tubular component 10 of a battery cell illustrating theinvention. Component 10 embodies porous tubular body 12 which functionsas a separator. Separator 12 is filled with a positive active material14. This may be the material commonly employed as a porous coating for apositive electrode or grid. A metal wire or rod 16 is then inserted inthe active material 14 of component 10 to function as the positiveelectrode. Normally, a complete cell in a battery will have a negativeelectrode on each side of component 10 or a series of such components.

Typically, a series of components 10 are combined to form an integratedelectrode. The series may, for example, number 15-20. The individualelectrodes 16 may be connected in known manner to form the integratedelectrode.

FIG. 2 is a schematic, partial view of an integrated electrode. TheFIGURE shows three components 10 electrically connected by a metal bar18 to produce integrated electrode 20. The opposite ends of thecomponents 10 may be held in a support member, for example, a moldedplastic holder 22.

FIG. 3 is an exploded side view illustrating an alternative, two-piececonstruction for an individual tubular component 30. In component 30,the ceramic separator takes the form of channeled, semi-cylindricalbodies 32 which may be identical in shape and material. Channels 34 ofbodies 32 are filled with positive active material 36 corresponding tothat shown in FIG. 1 at 14. Likewise, metal electrode member 38 isembedded in material 36. Bodies 32 are then sealed together to form acomponent 30 corresponding to component 10.

A series of components 30 may then be assembled to form an integratedelectrode in the manner described above. It will be appreciated that agauntlet-type construction may be produced by molding bodies havingmultiple, parallel channels, rather than a single channel as shown.

FIG. 4 is a perspective view showing tubular component 30 as a unitarybody formed by sealing together bodies 32.

SPECIFIC EMBODIMENTS

Development work has been largely carried out with our preferredmaterials, extruded alumina, mullite, or alumina/mullite mixtures. Thesematerials have been mixed with graphite prior to extrusion. As notedearlier, the graphite burns out of the extruded material to providebodies with improved porosities.

TABLE I shows batch compositions in parts by weight for a series ofmixtures which, when extruded and fired, provide bodies composed of 33%mullite and 67% alumina.

                  TABLE I                                                         ______________________________________                                        Batch                                                                         Materials  1       2      3    4    5    6    7                               ______________________________________                                        Platelet clay                                                                            16.66   14.13  11.63                                                                              9.14 7.80 5.82 4.16                            Stacked clay                                                                             5.54    4.71   3.88 3.05 2.49 1.94 1.39                            Calcined clay                                                                            27.61   23.47  19.34                                                                              15.19                                                                              12.42                                                                              9.66 6.90                            Alumina    50.73   42.67  35.16                                                                              27.63                                                                              22.60                                                                              17.58                                                                              12.56                           Graphite   --      15     30   45   55   65   75                              Methyl cellulose                                                                         3       3      3    3    3    3    3                               Dispersant 0.7     0.7    0.7  0.7  0.7  0.7  0.7                             Water      27.5    27.5   27.5 27.5 27.5 27.5 27.5                            ______________________________________                                    

The basic batches, prior to addition of graphite, were originallydesigned for preparation of support substrates exposed to temperaturecycling. Accordingly, combinations of platelet, stacked and calcinedclays (kaolin) were employed to control expansion effects by crystalorientation. The thermal expansion effects of the different clays, notof significance here, are explained in detail in U.S. Pat. No. 3,885,977(Lachman et al.)

TABLE II shows batch compositions in parts by weight for a similarseries of materials which, when extruded and fired, produce aluminabodies of varying porosity.

                  TABLE II                                                        ______________________________________                                        Batch                                                                         Materials    8      9          10   11                                        ______________________________________                                        Alumina      100    75         50   25                                        Graphite     --     25         50   75                                        Methyl cellulose                                                                           3      3          3    3                                         Dispersant   0.7    0.7        0.7  0.7                                       Water        27.5   27.5       27.5 27.5                                      ______________________________________                                    

TABLE III shows properties for fired, porous bodies produced from thebatches shown in TABLES I and II.

                  TABLE III                                                       ______________________________________                                                 MOR            Porosity                                                                              Pore Size                                     Example  Mpa (psi)      (%)     (Microns)                                     ______________________________________                                        1          51.2   (7400)    40.4  0.49                                        2          27.6   (4010)    50.7  0.60                                        3          14.9   (2160)    58.6  1.17                                        4          6.7    (973)     68.6  3.88                                        5          3.0    (431)     72.5  5.30                                        6          1.75   (253)     76.8  6.64                                        7          1.6    (232)     82.1  8.43                                        8          28.7   (4180)    40.3  1.07                                        9          7.4    (1080)    57.1  1.91                                        10         3.1    (456)     60.2  10.21                                       11         <0.7   (<100)    85.0  13.93                                       ______________________________________                                    

We claim:
 1. In a deep discharge, rechargeable battery assembly, theimprovement comprising a tubular, porous, ceramic separator for apositive electrode, the separator having a porosity greater than 50%. 2.In a deep-discharge battery assembly in accordance with claim 1, theimprovement comprising a tubular separator wherein the ceramic isselected from alumina, mullite and mixtures thereof.
 3. In adeep-discharge battery assembly in accordance with claim 1, theimprovement comprising a tubular separator wherein the separator has aporosity greater than 60%.
 4. In a deep-discharge battery assembly inaccordance with claim 1, the improvement comprising a tubular separatorcomprising two annular, semi-cylindrical bodies of porous ceramic sealedtogether at their side peripheries.
 5. In a deep-discharge batteryassembly, the improvement comprising a tubular, porous, ceramicseparator having a porosity of greater than 50%, a positive activematerial filling the interior of the separator, and a centralizedconductor embedded and extending from the positive active material. 6.In a deep discharge battery assembly in accordance with claim 5, theimprovement comprising a tubular, ceramic separator composed of amaterial selected from alumina, mullite, and mixtures thereof.
 7. In adeep discharge battery assembly in accordance with claim 5, theimprovement comprising a tubular ceramic separator having a porositygreater than 60%.
 8. In a deep discharge battery assembly in accordancewith claim 5, the improvement comprising a tubular, ceramic separatorformed as two annular, semi-cylindrical bodies, each body filled with apositive active material, a centralized conductor embedded in the activematerial and the two bodies sealed together.
 9. In a deep dischargebattery assembly in accordance with claim 5, the improvement comprisinga centralized conductor connected to a terminal common to a plurality ofindividual cells.
 10. In a deep-discharge battery assembly, comprising aplurality of individual positive electrodes held in a mounting andhaving a common terminal, each individual positive electrode comprisinga tubular separator having a positive active material filling theinterior of the separator and a centralized conductor embedded in andextending from the positive active material, the improvement whereby thetubular separator is a porous, ceramic body having a porosity greaterthan 50%.
 11. In a deep-discharge battery assembly in accordance withclaim 10, the improvement wherein the porous, tubular separator iscomposed of a material selected from alumina, mullite and mixturesthereof.
 12. In a deep-discharge battery assembly, in accordance withclaim 10, the improvement wherein the tubular separator in at least oneindividual, positive electrode is formed as two annular,semi-cylindrical bodies, each body is filled with a positive activematerial, an electrical conductor is embedded in the active material andthe two bodies are sealed together.
 13. In a deep-discharge, lightweight battery assembly, in accordance with claim 10, the improvementwhere the porous, ceramic separator provides a structural support forthe battery.
 14. In a deep-discharge battery assembly in accordance withclaim 10, the improvement wherein the battery is a lead-acid battery.15. In a deep-discharge battery assembly in accordance with claim 10,the improvement wherein the positive plate is a series of parallel,porous tubes.
 16. In a deep-discharge battery assembly in accordancewith claim 10, the improvement wherein the series of parallel, poroustubes are joined together in a gauntlet construction.
 17. In adeep-discharge battery assembly in accordance with claim 10, theimprovement wherein the porosity in the separator is sufficiently largethat the impressed voltage, to obtain a total energy output of 185Ah/lb., is below 2.7 volts.
 18. In a deep-discharge battery assembly inaccordance with claim 1, the improvement comprising a tubular separatorcomprising two unitary bodies of porous ceramic, each having multipleparallel semi-cylindrical channels and being sealed together at thechannel peripheries.